An electrochromic glass unit uses electrochromic glass that can change transmissivity with the application of electric current and voltage. The change of transmissivity typically relies on a reversible oxidation of a material. Electrochromic glass units can darken at the press of a button or other triggering event and are also often used in automobile rearview mirrors to reduce reflective glare. Controllers or drivers for electrochromic devices generally apply voltage and current of one polarity to charge the device and decrease optical transmissivity and the opposite polarity to discharge the device and increase the optical transmissivity.
The transmissivity change for current systems is relatively slow and non-uniform. Gradual, non-uniform coloring or switching is a common problem associated with large area electrochromic devices. This problem, commonly referred to as the “iris effect,” is typically the result of the voltage drop through the transparent conductive coatings providing electrical contact to one side or both sides of the device. For example, when a voltage is initially applied to the device, the potential is typically the greatest in the vicinity of the edge of the device (where the voltage is applied) and the least at the center of the device; as a result, there may be a significant difference between the transmissivity near the edge of the device and the transmissivity at the center of the device. Over time, however, the difference in applied voltage between the center and edge decreases and, as a result, the difference in transmissivity at the center and edge of the device decreases.
In addition, the current systems tend not to be robust as desired. Faradaic losses in reversible electrochromic devices can degrade the performance of reversible electrochromic devices. These faradaic losses can, in turn, result in a corresponding change in the oxidation state of an electrochromic material in the electrochromic device. The faradaic losses can occur in the electrochromic material that becomes optically less transmissive in its electrochemically oxidized state, the electrochromic material that becomes optically less transmissive in its electrochemically reduced state, or both. Over time and repeated cycling, the accumulated faradaic losses can cause a drift in the range of optical transmissivities achievable for the device within the desired operating voltage range for the device. The change of oxidation state and cell potential within the device can change due to many factors such as leakage current, spurious oxidation of materials, and aging of components.
What is therefore desired is a driver for an electrochromic device that can provide a power supply for fast and uniform switching, and methods for doing so repeatedly in a variety of conditions. Furthermore, an electrochromic device driver and methods that provide functionality to improve the robustness of the system, is also desirable. It is within this context that the embodiments arise.